Added a note about N+1 problem and form collections

[javiereguiluz]

Fixes #3374 and #4323.

[wouterj]

Please note that while joining seems like a performance optimalization, it makes hydration of doctrine way more complex for *..n relations. This often reduces performance instead of optimizing it: https://tideways.com/profiler/blog/5-doctrine-orm-performance-traps-you-should-avoid

[javiereguiluz]

@wouterj I read the article you linked, but decided to merge this anyways. You can see that even the article contradicts itself because in (2) they recommend to not use joins ... but in (5) they recommend to use joins.

This is a tricky issue ... but at least we now display a warning message about this so people can investigate on their own. Thanks.